With widespread use of portable terminal equipment in recent years, digital signal processing for processing a large amount of data such as voice/sound and images at high speed has become more important. An SRAM capable of high-speed access processing occupies an important place as a semiconductor memory device to be mounted on such portable terminal equipment.
On the other hand, a multiple-port semiconductor memory device capable of independent data writing and reading using each port has attracted attention, and an SRAM having multiple ports has increasingly been demanded.
In the case of dual ports representing one type of multiple ports, data can simultaneously be written and read through two ports.
In an SRAM having such dual ports, however, for example, particularly if data writing to the same row is performed at a time, generally, high load may be imposed on a memory cell and generally, such a command input may exceptionally be prohibited.
In this regard, Japanese Patent Laying-Open No. 07-141859 discloses a simplified method of sensing simultaneous access to the same row.
Here, when simultaneous access to the same row is made, though simultaneous data writing to the same row is prohibited, simultaneous data reading can be performed without any difficulty.    Patent Document 1: Japanese Patent Laying-Open No. 07-141859